Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k

[Greg Kurz]

On Thu, 7 Oct 2021 16:42:49 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> On Thu, Oct 07, 2021 at 02:51:55PM +0200, Christian Schoenebeck wrote:
> > On Donnerstag, 7. Oktober 2021 07:23:59 CEST Stefan Hajnoczi wrote:
> > > On Mon, Oct 04, 2021 at 09:38:00PM +0200, Christian Schoenebeck wrote:
> > > > At the moment the maximum transfer size with virtio is limited to 4M
> > > > (1024 * PAGE_SIZE). This series raises this limit to its maximum
> > > > theoretical possible transfer size of 128M (32k pages) according to the
> > > > virtio specs:
> > > > 
> > > > https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#
> > > > x1-240006
> > > Hi Christian,
> > > I took a quick look at the code:
> > > 


Hi,

Thanks Stefan for sharing virtio expertise and helping Christian !

> > > - The Linux 9p driver restricts descriptor chains to 128 elements
> > >   (net/9p/trans_virtio.c:VIRTQUEUE_NUM)
> > 
> > Yes, that's the limitation that I am about to remove (WIP); current kernel 
> > patches:
> > https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss@crudebyte.com/
> 
> I haven't read the patches yet but I'm concerned that today the driver
> is pretty well-behaved and this new patch series introduces a spec
> violation. Not fixing existing spec violations is okay, but adding new
> ones is a red flag. I think we need to figure out a clean solution.
> 
> > > - The QEMU 9pfs code passes iovecs directly to preadv(2) and will fail
> > >   with EINVAL when called with more than IOV_MAX iovecs
> > >   (hw/9pfs/9p.c:v9fs_read())
> > 
> > Hmm, which makes me wonder why I never encountered this error during 
> > testing.
> > 
> > Most people will use the 9p qemu 'local' fs driver backend in practice, so 
> > that v9fs_read() call would translate for most people to this 
> > implementation 
> > on QEMU side (hw/9p/9p-local.c):
> > 
> > static ssize_t local_preadv(FsContext *ctx, V9fsFidOpenState *fs,
> >                             const struct iovec *iov,
> >                             int iovcnt, off_t offset)
> > {
> > #ifdef CONFIG_PREADV
> >     return preadv(fs->fd, iov, iovcnt, offset);
> > #else
> >     int err = lseek(fs->fd, offset, SEEK_SET);
> >     if (err == -1) {
> >         return err;
> >     } else {
> >         return readv(fs->fd, iov, iovcnt);
> >     }
> > #endif
> > }
> > 
> > > Unless I misunderstood the code, neither side can take advantage of the
> > > new 32k descriptor chain limit?
> > > 
> > > Thanks,
> > > Stefan
> > 
> > I need to check that when I have some more time. One possible explanation 
> > might be that preadv() already has this wrapped into a loop in its 
> > implementation to circumvent a limit like IOV_MAX. It might be another "it 
> > works, but not portable" issue, but not sure.
> >
> > There are still a bunch of other issues I have to resolve. If you look at
> > net/9p/client.c on kernel side, you'll notice that it basically does this 
> > ATM
> > 
> >     kmalloc(msize);
> > 

Note that this is done twice : once for the T message (client request) and once
for the R message (server answer). The 9p driver could adjust the size of the T
message to what's really needed instead of allocating the full msize. R message
size is not known though.

> > for every 9p request. So not only does it allocate much more memory for 
> > every 
> > request than actually required (i.e. say 9pfs was mounted with msize=8M, 
> > then 
> > a 9p request that actually would just need 1k would nevertheless allocate 
> > 8M), 
> > but also it allocates > PAGE_SIZE, which obviously may fail at any time.
> 
> The PAGE_SIZE limitation sounds like a kmalloc() vs vmalloc() situation.
> 
> I saw zerocopy code in the 9p guest driver but didn't investigate when
> it's used. Maybe that should be used for large requests (file
> reads/writes)?

This is the case already : zero-copy is only used for reads/writes/readdir
if the requested size is 1k or more.

Also you'll note that in this case, the 9p driver doesn't allocate msize
for the T/R messages but only 4k, which is largely enough to hold the
header.

        /*
         * We allocate a inline protocol data of only 4k bytes.
         * The actual content is passed in zero-copy fashion.
         */
        req = p9_client_prepare_req(c, type, P9_ZC_HDR_SZ, fmt, ap);

and

/* size of header for zero copy read/write */
#define P9_ZC_HDR_SZ 4096

A huge msize only makes sense for Twrite, Rread and Rreaddir because
of the amount of data they convey. All other messages certainly fit
in a couple of kilobytes only (sorry, don't remember the numbers).

A first change should be to allocate MIN(XXX, msize) for the
regular non-zc case, where XXX could be a reasonable fixed
value (8k?). In the case of T messages, it is even possible
to adjust the size to what's exactly needed, ala snprintf(NULL).

> virtio-blk/scsi don't memcpy data into a new buffer, they
> directly access page cache or O_DIRECT pinned pages.
> 
> Stefan

Cheers,

--
Greg

pgpl7uMlbgxXL.pgp
Description: OpenPGP digital signature